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Engineers guided NASA's twin GRAIL lunar gravity probes into a ridge near the moon's north pole Monday, using the mission's final moments for technical experiments and honoring the legacy of late astronaut Sally Ride.

Artist's concept of the GRAIL satellites at the moon. Credit: NASA/JPL-Caltech

The two satellites, each about the size of a washing machine, hit the moon at about 5:29 p.m. EST (2229 GMT). Nicknamed Ebb and Flow, the craft were expected to strike the moon's surface about 30 seconds and one-and-a-half miles apart.

"Impact in 3, 2, 1, 0," a controller said over radio loops. Mission controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Lockheed Martin's operations center in Denver then lost the radio signal from each GRAIL spacecraft, indicating the satellites had smashed into the moon.

A round of applause broke out as officials confirmed the mission's ending.

Maria Zuber, GRAIL's principal investigator from the Massachusetts Institute of Technology, said NASA has approved naming the impact site for the late astronaut Sally Ride, who led the mission's MoonKam payload, a camera on each spacecraft designed for outreach to middle school students.

"Ebb and Flow have removed a veil from the moon, and removing this veil will enable discoveries about the way the moon formed and evolved for many years to come," Zuber said.

Ride's sister, Rev. Bear Ride, was in the JPL control room for GRAIL's impact. She said she was "appreciative" and "thrilled" the GRAIL team honored her sister.

"It's really cool now to think when you look up at the moon, there's this little corner of the moon named after Sally," Ride said.

Controllers choreographed the impact after GRAIL completed its science mission Friday, when the satellites fired rocket thrusters to aim for a ridge near Goldschmidt crater in the moon's northern hemisphere.

Less than one hour before they hit the moon, the satellites each emptied their fuel tanks with another engine firing, called a burn to depletion. The experiment will provide engineers with data on how much propellant was left aboard the spacecraft.

"We're going to use this data to compare with estimates of what the fuel remaining was," said David Lehman, GRAIL project manager at JPL. "It's very rare for a mission to be able to do a burn to depletion. That tells you exactly how much fuel was remaining."

Satellites are not equipped with fuel gauges, so engineers must estimate propellant quantities by other means, including pressure readings and the subtraction of burned fuel as it is consumed.

With real engineering data generated by the depletion burns, officials can verify the accuracy of their fuel predictions and maximize the time future missions can continue operating.

The $471 million Gravity Recovery and Interior Laboratory, or GRAIL, mission mapped the moon's gravity field, shedding light on the lunar interior and revealing the moon's crust is thinner than predicted.

GRAIL also found buried dikes underneath the lunar surface. The features - likely composed of ancient solidified lava - indicate the moon's size grew soon after it formed.

The mission's two satellites flew in formation around the moon since January, bouncing Ka-band radio signals between themselves to precisely measure their range. An on-board processor transmitted the radiometric data to the ground, where scientists are using the information to create gravity maps.

The mission worked by constantly measuring the distance between the two satellites as they orbited the moon. The tug of gravity from a surface or underground mass of rock registered in the range measurements between the spacecraft.

Because variations in the moon's gravity are so small, the GRAIL satellites could detect changes in their range as little as the width of a human hair, according to Sami Asmar, GRAIL project scientist at JPL.

"The knowledge of the gravitational field is now so accurate that, in the future, NASA and commercial missions will be able to navigate much more accurately [when] we land or fly over the moon," said Charles Elachi, director of JPL.

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